1. Field of the Invention
The present invention relates to an oil dynamic pressure bearing device formed from dynamic pressure grooves on at least one side of the opposing surfaces between a shaft and a bearing where oil is filled between the minute gap between the shaft and the bearing, and its manufacturing method.
2. Description of Related Art
As a bearing device for a hard disk drive motor, for example, there are considerations to use an oil dynamic pressure bearing device which has dynamic pressure grooves formed on at least one side of the opposing surfaces between the shaft and the bearing where oil is used as a lubricant filling the minute gap between the opposing shaft and bearing. For example, in motors for hard disk drives, a helical groove is formed on the outside of the dynamic pressure groove to prevent the oil from scattering from the bearing section because oil could render the hard disk unusable. Thus, the helical groove enhances the reliability of the oil dynamic pressure bearing and to extends the service life of the motor.
FIG. 3 is a developed view of a conceptual structure of a conventional bearing. Dynamic pressure generation grooves 3 are formed on two places in the shaft direction of the outer circumference of the shaft 1. This shaft 1 is inserted into the bearing hole of the sleeve-shaped bearing 2, forming a minute gap between the outer circumference of the shaft 1 and the bearing hole of the bearing 2.
Oil is used as lubricant to fill the minute gap. The oil generates dynamic pressure as the shaft 1 rotates around the bearing 2. This dynamic pressure allows the shaft 1 to be supported by the bearing 2 in a non-contact manner without physical contact. Helical (spiral) grooves 4 are formed on the bearing hole of the bearing 2 on the outside of the two dynamic pressure generation grooves 3. Oil is forced inward on the bearing 2 through the pumping force generated by the helical grooves 4, thus preventing the oil from scattering outside.
However, while available technologies explained above may be able to achieve the prescribed objective in terms of the reliability in preventing the scattering of oil when the motor turns, it will be difficult to prevent the wear and burning at the dynamic pressure bearing section caused by the contact between the shaft and the bearing when the motor is started, stopped or when the position of the motor is changed.
Also, there has been proposed a technique to prevent the break in oil film on the dynamic pressure bearing section. In the proposed technique, either the bearing or the rotating shaft is formed by a sintered oil-retaining bearing while the other is formed into a random rough surface. While this technique is applicable to a sintered oil-retaining bearing, the random rough surface that has no set directionality does not work effectively in dynamic pressure bearings of the type whose dynamic pressure is obtained from a non oil-retaining type dynamic pressure groove.
The present invention has been made to solve the problems explained in conventional technologies described above. The present invention thus pertains to providing an oil dynamic pressure bearing device and a method for manufacturing the oil dynamic pressure bearing device, which prevents wear or burning by the contact between the shaft and the bearing even when the rotation starts or stops, or there is a change in position of the bearing, by enhancing the fit of the shaft with respect to the bearing and the oil retaining properties as a lubricating agent.
In accordance with an embodiment of the present invention, an oil dynamic pressure bearing device has a dynamic pressure groove that generates dynamic pressure on at least one side of the opposing shaft and bearing, in which oil is filled in the minute gap between the shaft and the bearing as lubricant fluid, wherein grinding scratches that are formed in the direction of the circumference as the dynamic pressure bearing section is ground are provided independently throughout the circumference.
In the present embodiment, when the dynamic pressure bearing section, on which the grinding scratches are formed, is traced in the shaft direction, which is perpendicular to the direction in which the scratches are formed, the degree of surface roughness Ry may preferably be Ry=about 0.02 s to 3.2 s.
In the present embodiment, a dynamic pressure groove may be formed only on one side of either the shaft or the bearing of the dynamic pressure bearing section and the grinding scratches may be formed only on the other side. However, the dynamic pressure groove may preferably be formed only on the bearing side of the dynamic bearing section, and the grinding scratches on the shaft side, because it does not affect the formation of the dynamic pressure groove.
Also, grease, highly viscose oil or solid lubricant may preferably be applied on the dynamic pressure bearing section on which the grinding scratches are formed.
Moreover, in the present embodiment, the grinding scratches in the direction of the circumference may be formed independently from one another and arranged through the entire circumference, each of the grinding scratches continuously extending one third or more of the circumference in the direction of the circumference.
The grinding scratches in the circumference direction are designed to facilitate the flow of oil to the direction of the circumference, the direction of the rotation. Therefore, the effect of the present invention cannot be expected if short scratches are formed too far apart. It will also make manufacturing more difficult. In a preferred embodiment, the grinding scratch may be shaped in concentric form or spiral form. However, when a plurality of scratches, each of the scratches extending one third or more of the circumference, are formed independently from one another, even though they do not extend longer than one circle around the circumference, the same effect that may be obtained by scratches in concentric form can be obtained if the scratches in total are formed around the entire circumference of the shaft. It is noted that this does not mean that the grinding scratches must be in a concentric form.
Also, it is necessary to exert caution so as not to make the length of the grinding scratch too long in the shaft direction if the grinding scratch is formed in the direction of the circumference into a spiral form. Notably, if the spiral form scratch extends to the outside the bearing section in the shaft direction, caution must be exerted to prevent the oil from being led to the outside of the bearing section by the spiral grinding scratch. Because of this, it is best to sever the spiral-form grinding scratch to make them independent so they do not connect at length. Moreover, if the spiral scratch extends to the outside the bearing section in the shaft direction, the spiral scratch may preferably be formed such that the flow direction of the oil should be formed in the direction toward the interior of the bearing by taking into consideration the direction of rotation.
Moreover, in accordance with another embodiment of the present invention, in a method for manufacturing an oil dynamic pressure bearing device that has a dynamic pressure groove that generates dynamic pressure on at least one side of the opposing shaft and bearing, in which oil is filled in the minute gap between the shaft and the bearing as lubricant fluid, wherein grinding scratches are formed on the dynamic pressure bearing section in a direction of the circumference independently from one another throughout the circumference by grinding the dynamic pressure bearing section.
In the embodiment described above, the grinding scratches may be formed with lapping sheet, abrasive cloth containing abrasive particles or sandpaper, or with center-less grinding.
Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the invention.